1. Technical Field
The present disclosure relates to systems and methods for determining the attitude of an operational satellite, and in particular to a system and method for determining the attitude of a satellite in harsh environments using crosslink reference signals.
2. Description of the Related Art
Communications satellites are artificial satellites that are deployed in space for the purpose of telecommunications. Depending on the desired mission, constellations of such satellites are deployed in the appropriate orbit, which may include geostationary, mid-Earth, low-Earth, polar, equatorial, or Molniya orbits. One of the advantages of such satellite systems is that they can be used to implement worldwide communications at low cost.
In many cases, the satellite communications system is required to provide communications using spot beams and/or laser beams with high precision. This requires that the satellite be capable of determining its attitude to a high degree of accuracy for spot beam pointing and laser beam acquisition and tracking. Ordinarily, such determinations can be made using star sensors and the like. However, such sensors may not be effective in the presence of either localized or distant man-made nuclear environments. Other sensors, such as nuclear survivable MMS Earth Sensors (Multi-Mission Earth Sensors) are available. However, such sensors do not meet the satellite attitude measurement accuracy requirements for some applications.
Other methods for determining satellite attitude have been proposed. For example, U.S. Pat. No. 5,959,576 discloses a system that determines the host satellite's attitude using the global positioning system (GPS) receiver signals from the host satellite and signals provided through other satellites through laser communications links. The signals consist of ephemeris data from GPS receivers in the other satellites. However, such data is not used in the satellite's existing attitude determination system to enhance the accuracy of the satellite's determination of its attitude when Earth/Sun sensors are used with continuously running inertial sensors (such as gyros and accelerometers) or when star trackers are used with such inertial sensors. Hence, the GPS receiver signals do not provide results to acceptable accuracy for many missions, particularly in harsh environments.
U.S. Pat. No. 6,219,593 determines the host satellite attitude using laser communications crosslink data and a well-known TRIAD algorithm, which is similar to that which is disclosed in U.S. Pat. No. 5,959,576. Methods to acquire the crosslink signal are also disclosed. U.S. Pat. No. 6,778,886 discloses a laser transmission/reception system providing range data measurements for a host satellite to determine its position from at least three other satellites. However, this data in these systems is not used in the satellite's existing attitude determination system to enhance the accuracy of the satellite's determination of its attitude, and consequently, this system also does not meet the accuracy requirements for many missions, particularly in harsh environments.
What is needed is a system and method for using crosslink data to improve the satellite's determination of its attitude, particularly in harsh environments. The present invention satisfies this need.